The present invention relates generally to heaters useful in thermal processing apparatuses, more particularly to heaters used in the rapid thermal processing of semiconductor substrates, and especially to an apparatus and method for heating semiconductor substrates during chemical-vapor-deposition (CVD) processes, including epitaxial reaction processes. Accordingly, it will be understood that when reference is made in the remainder of this application to CVD or epitaxial deposition processes, these are merely prime examples of the range of thermal processes to which the teachings of the present invention are applicable.
The commercial production of semiconductor devices has in recent times been placed under increasing pressure to reduce the cost per device. This in turn has required new measures to increase the efficiency of epitaxial processing methods so that they yield higher throughput of acceptable devices at a lower cost per device. One important recent development in this regard is a compact, double-dome reactor which achieves increased processing speed and reduced consumption of the gases used in the epitaxial reaction.
Such an apparatus is fully detailed in a pending U.S. patent application entitled "Double-Dome Reactor for Semiconductor Processing", Ser. No. 07/491,416, filed Mar. 9, 1990, and commonly assigned with the present application. The reactor system utilized in this cited application is efficient enough to permit economic processing of even a single wafer per processing operation.
The central concepts of this double-dome reactor system may be summarized as follows: (1) support the substrate on a thin, low-mass susceptor for rapid heating and cooling; (2) enclose the substrate and susceptor in a compact housing having a pair of transparent domes, each dome covering one face of the susceptor-substrate combination; (3) provide conduits for reactant gases to enter and exit the housing; (4) provide a pair of radiant heaters arranged to project thermal radiation through each of the transparent domes to uniformly heat each face of the susceptor-substrate combination.
While the reactor system described in the above patent double dome application has proven very efficient in reducing processing cost and increasing throughput, work has continued on further improvements in these regards. The radiant heaters shown and described in the double dome patent application are of the sort which uses a pair of concentric arrays of heater lamps in a simple cylindrical reflector housing, one lamp array near the outer periphery and the other nearer the center of the cylindrically symmetric arrangement. The two arrays use different types of lamps having different radiation patterns, as shown in the drawing of the double dome patent application.
Such an arrangement achieves good radial uniformity of thermal radiation from the center to the edge of the substrate, while rotation of the substrate about the axis of cylindrical symmetry effectively cancels any azimuthal non-uniformities of radiation. However, achieving similar radial uniformity of thermal radiation without requiring two concentric arrays of heater lamps of two different types would be very desirable, since the cost of the heaters could be reduced.